The invention relates to a controller for an electromagnetic actuator for driving a valve of an engine mounted on such apparatus as an automobile and a boat.
A valve driving mechanism having an electromagnetic actuator has been known and called a magnetic valve. An electromagnetic actuator typically includes a moving iron or an armature which is placed between a pair of springs with given off-set load so that the armature positions at an intermediate part of a pair of electromagnets. A valve is connected to the armature. When electric power is supplied to the pair of electromagnets alternately, the armature is driven reciprocally in two opposite directions thereby driving the valve. Conventionally, the driving manner is as follows.
1) The magnetic attraction power that one of the electromagnets provides to the armature overcomes rebound power by the pair of springs and attracts the armature to make it seat on a seating position. The armature (valve) is released from the seating position by such a trigger as suspension of power supply to the electromagnet, and starts to displace in a cosine function manner by the force of the pair of springs.
2) At a timing according to the displacement of the armature, an appropriate current is supplied to the other electromagnet to produce magnetic flux which generates attraction force.
3) The magnetic flux rapidly grows as the armature approaches the other electromagnet that is producing the magnetic flux. The work by the attraction power generated by the other electromagnet overcomes the sum of (i) a small work by the residual magnetic flux produced by the one electromagnet which acts on the armature to pull it back and (ii) a mechanical loss which accounts for a large portion of the sum of work. Thus, the armature is attracted and seats on the other electromagnet.
4) At an appropriate timing as the armature seats, a constant current is supplied to the other electromagnet to hold the armature in the seated state.
Application of such valve driving mechanism to intake and exhaust valves of an engine mounted on an automobile is proposed. The valve driving mechanism may not appropriately operate depending on driving conditions of an engine. When the engine is operating in a higher load condition, the exhaust valve cannot be easily opened. This is because high-pressure gas within the cylinder of the engine is applied to the valve during a period from the combustion stroke to the exhaust stroke.
More specifically, in operation of opening the valve, when the armature is released from one of the electromagnets (hereinafter, referred to as “valve-closing electromagnet”) and is moving toward the other of the electromagnets (hereinafter, referred to as “valve-opening electromagnet”), energy stored in the springs is converted into mechanical work of the valve against the exhaust gas pressure. In the case where the armature is held in a neutral position given by the springs and the neutral position is located at the midpoint of the pair of electromagnets, the armature may not sufficiently approach the valve-opening electromagnet. In order to attract the armature and open the valve, it is required to apply a large amount of electric power to the valve-opening electromagnet. This means that the operation for opening the valve consumes a large amount of electric power.
One of conventionally proposed schemes for opening an exhaust valve more easily in high-load conditions is to move the neutral position of the armature of the exhaust valve slightly toward the valve-opening electromagnet beforehand. The offset of the neutral position increases the potential energy of the springs when the armature is seated on the valve-closing electromagnet. The movement of the armature when the valve-opening operation starts is accelerated. The armature sufficiently approaches the valve-opening electromagnet even if the mechanical work against the exhaust gas is done.
However, the scheme of moving the neutral position of the armature has a problem. In low-load conditions such as idling state, the gas pressure within the cylinder when the armature is released from the valve-closing electromagnet is not so strong as in high-load conditions. The mechanical work by the valve against the exhaust gas decreases. The potential energy of the springs when the valve-opening operation starts is excessive. When the armature is released in such condition, the armature may vigorously move toward the valve-opening electromagnet. Even if electric power is not supplied to the valve-opening electromagnet, the armature may collide with the yoke of the valve-opening electromagnet, thereby generating a large collision noise.
There is a need for a controller for an electromagnetic actuator in which the valve, especially the exhaust valve, is opened without additional electric power in high-load conditions and in which collision of the armature with the yoke in low-load conditions is prevented.